1. Field of the Invention
The present invention relates to a communications system, and in particular to a heterogeneous system employing a number of different call control mechanisms and different address types.
2. Description of Related Art
In a conventional, homogeneous, communications network, such as the public switched telephony network (PSTN), customers have only one type of address (in this case their telephone number) and there is a single uniform call control mechanism which is built into the network. The call control mechanism is used for establishing and for terminating calls and for recognising, e.g., when a called party is busy. Increasingly, however, customers have access to a range of different network technologies, each with its own address type. For example, a customer might have in addition to a telephone number a conventional IP (Internet protocol) address, a multicast IP address and a URL (uniform resource locator). In general, each of these different address types has associated with it a respective call control protocol (where the term “call control” is used broadly to denote the means for establishing and terminating connections between different parties). For example, audio or visual communication between parties using conventional IP addresses commonly uses the H.323 protocol, whereas for communication between broadband ATM addresses a different protocol, B-ISDN (broadband-integrated services digital network), is used. In practice, the call control protocol which is used for a particular communication session tends to be determined by the party who initiates the session. If other parties later join a session, they are then restricted to using the addressing and call control capabilities determined by the initiating party.